To twirl or not to twirl, that is the question

There's two ways to stop a galaxy from collapsing. Either it can spin around, which flattens the gas and stars into a disc, or everything can be on chaotic, random orbits. The latter doesn't really work for gas, which is collisional and tends to do unpleasant things like shock and lose energy.

What about all those recent ultra diffuse galaxies ? If we want to know if they're dwarfs or giants, which will affect theories of galaxy formation, we'll need to know if they're spinning or supported by dispersion. This paper looks at a sample from simulations.

The paper gives a very good overview of the current state of play for UDGs, noting the interesting shenanigans going on with deviations from the Tully Fisher relation, the diversity of the sample, the different ways to measure their dimensions, and the different formation mechanisms proposed. Essentially there are two main ideas : they could form due to internal processes, or they could form in clusters (where most UDGs have been found) due to unique environmental processes. The later doesn't seem likely, or at least it certainly isn't sufficient, since we now know of isolated UDGs that have never even see a cluster.

The one major thing I don't like about this cluster is that they're unclear about their simulations and sample selection. I don't even like reading these bits of simulation papers and I normally just skim them, but even I found this too brief. It's important to know what the mass resolution is and what they mean when they say they selected an "isolated" sample - how you define isolation is basically arbitrary. They don't even say how many galaxies were in their simulation in total or how large a cosmological volume they simulated. Sure, I could consult the original simulation papers, but I shouldn't have to : these parameters are fundamental to this paper.

That aside, they find that almost exactly half of their sample of 38 isolated UDGs are dispersion dominated and the other half rotation supported. They tend to be gas rich (though "rich-most" is an ugly phrase indeed), though dispersion dominated galaxies are more gas poor. This is partly a selection effect, in that rotation is better at supporting more extended systems at any given mass. Also, their objects are not part of the high-spin tail of normal galaxies, as one of the most popular early papers claimed.

Interestingly, at these masses, supernovae feedback is expected to be highly efficient, but they find that this plays only a secondary role in determining whether a UDG is rotation or dispersion supported. The major driver appears to be how the gas accretes. If it happens to align with the existing rotation of a proto-galaxy, the accreting gas helps increase the rotation. This lets it maintain a steady accretion of gas without any destructive starbursts. If, however, the accreting gas is misaligned, then chaotic accretion leads to bursts of star formation and supernovae feedback that quickly reduces any further gas inflow. So rotationally supported systems tend to have later gas accumulation.

Prior to the lockdown I was writing a paper on gas observations of UDGs. Although the deviation from the Tully-Fisher of some objects is weird and interesting, quite a few don't show this : they look like normal rotating discs. So this paper makes a lot of sense to me, but I would have liked more information. How extended are these galaxies using different radius measurements ? What's their typical star formation rate - do they have much molecular gas ? How are they still accumulating gas when they're in isolation ? On the other hand, if they'd given all that I might have dismissed the paper for being too long and not read it at all...

NIHAO XXIV: Rotation or pressure supported systems? Simulated Ultra Diffuse Galaxies show a broad distribution in their stellar kinematics

In recent years a new window on galaxy evolution opened, thanks to the increasing discovery of galaxies with a low surface brightness, such as Ultra Diffuse Galaxies (UDGs). The formation mechanism of these systems is still a much debated question, and so are their kinematical properties.

The slightly surprising survival of spirals

Galaxy evolution is very similar to that of large, ruthless corporations : it's driven by merger after merger, until what started off as a small family shop becomes a huge international conglomerate that sells weapons specifically designed for killing babies living in third-world countries. Or something. A slightly more popular analogy is that galaxies are cannibals, but this is no less grisly as it still involves fat, bloated monster galaxies that mercilessly devour their smaller, weaker siblings.

Look, I've been stuck inside for a month. I've got to make my own entertainment. Stop judging me !

All that horror and questionable politics aside, there's a bit of a puzzle here. Merging is thought to be the most common way to turn spiral galaxies into ellipticals. But if that's so, how can there be any massive spirals around today ? Shouldn't their formation necessitate lots of mergers, making them far more likely to become ellipticals ?

The authors of this study look at one of the all-singing, all-dancing, "everything on" cosmological simulations to trace the evolution of the most massive spiral galaxies. This simulates a large cosmological volume, so unlike smaller simulations (which have to be focused on specific environments) it includes a whole range of evolutionary processes. It's also of extremely high mass resolution, enough to simulate even very modest dwarf galaxies. This means that from the perspective of the most massive spirals, which they concentrate on here, it should be complete.

They find that giant spirals are indeed rare. Of galaxies with more than 100 billion solar masses in stars, about 11% are spirals, in rough agreement with observations.

There are two different ways they can form. About 70% result from mergers... but not just any mergers. The mass ratio of the progenitors doesn't seem to matter much, but their gas content does. Since the gas is collisonal, the merger causes it to shock and collapse to a disc supported by the angular momentum imparted by the merging galaxy.

Even more persuasively, 98% of all massive spheroids which experience a gas-rich merger subsequently become discs, so gas-rich mergers appear to be a highly effective route for disc formation. And the discs tend to have had their last major merger more recently than spheroids, meaning there hasn't been much time for other, unsuitable mergers (or other processes) to destroy them. In addition, as the gas content of the simulated universe decreases over time, so does the fraction of galaxies which are massive discs. It's pretty clear that it's gas what done it.

Apart from these "rejuvenated discs", the remaining 30% are always discs. Presumably they initially form by mergers, but subsequently they get lucky and have a very quiet merger history. They mention an interesting idea was that such galaxies could be near to even more massive galaxies whose enormous gravity could protect their smaller brethren from incoming would-be mergers, but this doesn't appear to be the case for their sample : they seldom have a more massive companion. So for the most massive objects, at least, it's simply a matter of luck.

Whether this has any bearing on runaway capitalism or the culinary practises of uncontacted jungle tribes is left as an exercise for the reader.

Why do extremely massive disc galaxies exist today?

Galaxy merger histories correlate strongly with stellar mass, largely regardless of morphology. Thus, at fixed stellar mass, spheroids and discs share similar assembly histories, both in terms of the frequency of mergers and the distribution of their mass ratios.

Filament finding fun

Where do galaxies get beaten to death ? Is a galaxy falling into a cluster about to be torn limb from limb, or is it likely to be half-dead already ?

Of course, it probably varies considerably. We know that the processes at work in clusters - mainly tidal harassment and ram-pressure stripping - certainly can cause enormous damage. And we know that at least some galaxies falling into clusters look, for the time being, perfectly healthy. But of course the real question is what happens in a typical case, if there even is such a thing.

Since we can't wait around for billions of years to track individual galaxies, like most extragalactic problems we have to tackle this statistically. As we generally have rather small samples - a few hundred galaxies per cluster - we plot radial trends in galaxy properties (colour, gas content, morphology and so on) as clues to the general trends at work. The problem is that this biases us to looking for cluster-induced changes. What about processes at work more locally, or even occurring to galaxies before they've fallen into the cluster proper ? Such "pre-processing" might be very important, but would be easy to miss in radially-averaged plots.

Being outright annoyed by such seductively simple but potentially misleading techniques, my postdoc* Boris Deshev decided to try something more sophisticated and plot Voronoi maps. We can't avoid the need to bin the data due to our small sample size, but this method allows us to really see the 2D map of various properties instead of the 1D radial average. It's a sort of adaptive-gridding that's good for dealing with points that vary widely in density from place to place, whereas a uniform gridding would wipe out a lot of fine details.

* Can one own a postdoc ? Native Americans say you can't own land, but as far as I know they don't say anything about postdocs.

The main map in the paper is a Voronoi map of the fraction of star-forming galaxies. This does not show the neat, circular trend that you might expect if the cluster was dominating galaxy evolution. In earlier drafts, we saw a clear north-south filament of galaxies dominated by non star-forming objects, but sadly this largely faded from view with more accurate star formation measurements. But it re-appeared when looking at the specific star formation rate, that is, the star formation activity accounting for the mass of each galaxy.

It's even possible to show that those objects haven't had their star formation reduced recently - it must have happened in the relatively distant past (> 500 Myr ago). So it does appear that pre-processing is having a significant influence : part of the cluster is assembling along a filament in which galaxies have already had their star-formation activity reduced. This Voronoi mapping is a neat way to show things that radial plots cannot, though the disappearing filament is an important reminder that small differences can sometimes make a big difference to the result.

This cluster is also one of only two at this distance (about 2.6 billion light years) which have HI measurements. At that distance, the resolution of the observations isn't great. Even so, there are some intriguing hints of galaxies being caught in the act of gas loss, with their HI noticeably displaced, stretched, and shifted in velocity from their most likely parent galaxy. The main problem is identifying the parent galaxy : we can't be sure that it isn't actually some barely-visible little blob that's just about visible within the beam of the telescope. But a few cases look intriguing enough that it was certainly worth reporting, as no-one's detected this at this redshift before (two of them look pretty darn convincing to me). They'll be good targets for future observations at higher resolution, and there might be more candidates hiding in the data. Developing new ways to dig them out is ongoing.

Mapping the working of environmental effects in A963

We qualitatively assess and map the relative contribution of pre-processing and cluster related processes to the build-up of A963, a massive cluster at z=0.2 showing an unusually high fraction of star forming galaxies in its interior.

The dirt in the discs

What, another paper on dust ? "But Rhys", I hear you say, "you said you hated dust !". Well, I do - it's awful. Outside of His Dark Materials, dust is not the least bit interesting. I just can't bring myself to become interested in where it comes from or what it does. Come on - it's feckin' dust. Dust is something you get rid of. Every time someone mentions it, I immediately remember an advertising slogan for a hoover : "compress your dust, compress your worries". Almost as good as a jingle on a children's magazine, "I love horses, they're my friends !"... but I digress.

Anyway, what caught my eye about this paper was not the dust, but the detection of spiral structure in elliptical galaxies. Since such structures have been found in early-type galaxies in Virgo, this is a potentially interesting avenue for exploring galaxy evolution as a function of environment.

This paper looks at a sample of field ellipticals, using two deliberately similar populations : one with and one without gas (both HI and CO). They use the MegaCam instrument on the CFHT to produce colour maps, using reddening as an indication of dust content after controlling for the underlying stellar population (I also find myself wondering why both this and the previous paper never mention Herschel or other direct dust measurements). This shows a variety of different dust morphologies. Here's their main figure using the SDSS optical images :

And here's the same sample (with no attempt at all at keeping the scaling constant) using their dust maps, taken directly from the paper :

N = no dust; D = disc; R = ring; Ir = Irregular

Now to be fair, you can see some hints of these structures in the conventional images, but they're far clearer in the dust maps. NGC 3626 I find particularly interesting : although it certainly doesn't look like a typical elliptical in the optical map, the spiral dust structure is quite different again. And if you'd just shown me it's dust map, I'd have assumed it was a quite typical spiral shown with a weird colour scheme.

The main trend they note is that there are clear differences in dust morphology depending on gas content. The presence of any gas anywhere in the galaxy seems to be an excellent predictor of dust in the central regions, which it would be nice to know a bit more about... what's the physical connection between outer gas and inner dust ? Perhaps they say something in the appendix, but that's a billion pages long so I'm not going to read it. They also show that gas-rich galaxies are dominated by dust morphologies of spiral and irregular types, with no rings and a very few discs. Gas-poor galaxies have dust morphologies dominated by discs, rings and irregulars. Gas-rich galaxies tend to have large dust structures, whereas small dust structures are found in gas-poor galaxies.

Understandably, they put a lot of effort into comparisons of their study with others, both in terms of similarities and differences. In particular, their sample only deals with field galaxies, so cluster galaxies (which are almost unanimously gas-poor) may be different. But what does this tell us about galaxy evolution ? When some apparently early-type galaxies look so strikingly similar to late-types by looking at their dust, I think we have to wonder if this indicates a connection between the two. They speculate that the origin of the dust could be due to internal production (by stars, despite the low rate of star formation) and brought in externally, but honestly, that's boring. I think this result is sufficient to warrant asking bigger questions about the connections between galaxies of different morphologies. Does one type evolve into the other ? Do they have more similar formation mechanisms than previously suspected ? I dunno, but that to me is way more interesting than any amount of bloomin' dust.

Cold gas and dust: Hunting spiral-like structures in early-type galaxies

Observations of neutral hydrogen (HI) and molecular gas show that 50% of all nearby early-type galaxies (ETGs) contain some cold gas. Molecular gas is always found in small gas discs in the central region of the galaxy, while neutral hydrogen is often distributed in a low-column density disc or ring typically extending well beyond the stellar body.

The Virgo Cluster is a dirty, dirty place

And now we return to regular boring old science with nary a space vampire in sight.

One of the many, many wonderful things about the Virgo Cluster is that it lies behind a great big hole in Galactic dust. So we can measure the galaxies there without worrying too much that the foreground dust has made them appear significantly redder and fainter than they really are*. That's especially nice for me, because I can't stand dust. I just cannot bring myself to get interested in it. Sure, it may be an important component of star formation, but... come on, it's dust. Don't expect an insightful commentary, is what I'm saying.

* This is called extinction, presumably just to be confusing.

Anyway, through some very sophisticated modelling it is possible to correct for the reddening due to foreground dust. Once that was done, the authors looked at the extinction variation of Virgo galaxies that must be caused by extragalactic sources. Normally I'd probably be skeptical, but the trend is so darn clear - more reddening near the cluster centre with a very rapid drop off with radius - that
it looks to be pretty convincing, at least to a naive dust-aversion person like me.

They're even able to make a map of the extinction, albeit at low resolution, but you can clearly see something. Dunno what it is, but it's definitely there (the issue is you can only measure the dust by looking at galaxies which have been reddened by it, rather than detecting it directly). Broadly, they say it seems to follow the same distribution of the intracluster light, thought to result from stars that have been thrown out of their galaxies by tidal interactions. They estimate the total mass of dust at just 3 billion solar masses, which is about the same as the gas mass of a single large dwarf galaxy. Being able to detect this when spread over such an enormous area is pretty darn impressive.

The main thing I wonder about is survival. They say the expected lifetime for dust in the cluster environment is about 100 million years, which is not all that long. Are there likely to be enough tidal encounters pulling out sufficient dust to explain the observations ? There's not that much of it, but I would expect it to be pretty hard to remove since it should be mainly found in the inner regions of galaxies. I dunno. I might just be too naive on this, but it would certainly be interesting to know how much dust can be removed in an encounter and then get a handle on how often such encounters must occur. Which would likely end up as an exhausting project involving all kinds of ghastly physics with a result that may or may not be interesting to anyone. Such is life.

The GALEX Ultraviolet Virgo Cluster Survey (GUViCS) VIII. Diffuse dust in the Virgo intra-cluster space

We present the first detection of diffuse dust in the intra-cluster medium of the Virgo cluster out to $\sim$0.4 virial radii, and study the radial variation of its properties on a radial scale of the virial radius.

Behold April

In the words of the great Jeremy Paxman when he was forced to present a weather forecast, "Well what did you expect ? It's April", so it is for arXiv. Here's a brief round-up for those who missed it.


The Really Habitable Zone

Who cares if liquid water can exist on a planet orbiting a star ? The important thing is whether conditions are suitable for making a gin and tonic, a region "which might actually be worth existing on." Astronomers, they say, need alcohol, and the presence of astronomers is a good definition of civilisation. And a lack of gin might explain why a planet has a terrible atmosphere.

(They note also the possibility of "ginspermia", in which juniper bushes propagate through interstellar dispersal, but suggest that efficient harvesting for maximum gin productions means there are no spare bushes flying through space.)

How to define the parameters for the RHZ as opposed to the old BHZ (Boring Habitable Zone) ? They followed standard practise and made them up. This all involved the consumption of multiple "gins and tonic", which is not a typo : "You want multiple gins, not more tonic." Adorable.


Searching for Space Vampires

"It is a truth universally acknowledged, that a single human in possession of a good space telescope,
must be in search of a space vampire." A strong opener indeed. Apparently inspired by an xkcd webcomic which notes that reflecting telescopes can't see space vampires, the authors realised that not all telescopes these days use mirrors. The Transiting Exoplanet - sorry, Exovampire Survey Satellite uses lenses, so is an ideal tool for searching for spaceborne undead.

They note three prospects for space vampires : free-floating, to be examined in a forthcoming paper by Van Helsing et al.; already landed on Earth; tidally locked around M-dwarf stars. Why M dwarfs ? Because while vampires are susceptible to sunlight, they are clearly unaffected by firelight, which is typically not too much cooler than M dwarf stars. Presumably daylight on planets around such stars holds no fears for vampires there...

They then drew a profile of a vampire and a bat in Microsoft Paint and fed it through some transit photometry modelling package to see the expected dip in the light. Comparing to observations from TEvSS, they find there could be between 0 and 394400933 possible space vampire transit events, i.e. between 0 and 100% of all observations. They consider this to be a major breakthrough.


Conspiratorial Cosmology

Reality is just a conspiracy and "generally misleading", say the authors of this the most incoherent of the suggestions. Loosely based on the idea that if anyone figured out the meaning of existence, the Universe would disappear and be replaced with something more complex, and this has already happened, the authors develop the concept of inflationary imbecility. This is like regular inflation, only for stupidity instead of space.

But who are They who are behind it all ? Following the Chuck Norris theorem - that there's an easy way and a hard way - they suggest that They started with a simple Universe and have been gradually ramping up the complexity. Recent events comprise such things as the challenge of electing an unelectable president and controlling an uncontrollable virus.

And why is the Universe controlled by so much dark matter and dark energy and the like ? They solved this by employing occultism, in which a medium declared that it's due to the Fertile Neutrino (as opposed to sterile neutrinos, of course). This is, apparently, very fertile up to several "guinea pig units", and the ongoing production of fertile neutrinos causes the expansion of space. There follows a lengthy rant about the anthropic principle, philosophy, string theory, the importance of simulations, eventually concluded that we ourselves are the grand conspirators behind it all, somehow. I got as far as, "In order to make sure that we need all the energy we harvest, we will build our spaceships in form of giant SUVs which are constructed such that they have a decent wind-resistance even in the interstellar medium, and we will cause a greenhouse effect in the Galaxy so strong that Dyson-trees (Dyson, 1997) start to grow on molecular clouds" before deciding that the words, "please stop" had never been more appropriate and gave up.


And finally, overheard elsewhere :

Normal services will be resumed as soon as possible.

Slowly does it

And now back to our regular service on galaxy dynamics and whether or not MOND is a thing.

Galaxies typically have rotation curves which are approximately flat in their outer regions. They usually rise steeply in the innermost regions, often going a bit crazy at first, but then settle down to something that's basically flat and boring. Dwarf galaxies are an exception, often showing curves which are still (slowly) rising, possibly because the stars and gas don't probe the full dark matter halo. But even a few larger galaxies also show curves which are weakly rising or declining.

Recently we looked at massive galaxies which are rotating more quickly than expected, and whether these challenge alternative theories of gravity. It seems to me that they probably do, but critics are right to point out that it matters a great deal as to which value you use to define the rotation of a galaxy, i.e. the peak or the flat part of the curve.

This latest paper looks at galaxies with declining rotation curves. They choose their sample so as to avoid the usual causes of this, like interactions or having a strong bar or bulge in the centre (such that the mass of the innermost baryons becomes significant compared to the extended dark matter). They also select them to have well-resolved rotation curves, so it's not a data artifact or anything daft like that. And unlike the previous paper, they show all of their curves, rather than coyly hiding them away like in the last one.

What I cannot for the life of me understand are their quoted ratios of peak to outermost velocity. Looking at their rotation curves, I'd guestimate the ratios to be no more than a factor of two at the very most, but they give values of 10-40 ! I'm tempted to email them because I can't make sense of that, but the text has been translated from the original Russian so that might make things difficult.

When it comes to the inevitable Tully-Fisher relation (a comparison of rotation speed and luminous mass), they show that their galaxies agree with the standard relation if they use the peak velocity, but are significantly slower than the standard relation predicts if they use the flat velocity. This is exactly the opposite of what Milgrom said when criticising the fast rotators : these authors have done exactly as he suggested, and find the opposite sort of problem ! So the claim of MOND enthusiasts that the Tully-Fisher relation actually has some miniscule scatter if you get the measurements right looks extremely suspicious to me : galaxies actually seem to deviate in all directions, even very massive ones. And furthermore, since the peak velocities here do agree with the standard relation, that makes it rather unlikely that there's been some systematic error in the velocity estimate.

The nice thing about rotation curves, though, is that you're not limited by global relations like Tully-Fisher. You can directly compare a galaxy's observed and predicted rotation throughout its whole disc. For this sample, they find most galaxies agree with MOND's predictions, but not all*. A few require the galaxies to have significantly different stellar mass from the measured value, and/or a different acceleration constant - in the worst case by a factor of six. And it would be a pretty stupid theory indeed if you had to change a fundamental constant for each galaxy.

* And they're interesting curves in their own right : some look flat to me, while others are very clearly and consistently declining, and still others show sudden decreases and then remain flat further out.

There is some scope for a MOND rebuttal here. The galaxies are not all truly isolated (one is even in a cluster). The authors say they have no nearby massive companions capable of creating a significant external field effect, but it could be that they've interacted in the recent past and are still out of equilibrium (I don't know if anyone's modelled how this would affect rotation curves, but I'd be a bit surprised if this caused them to rotate more slowly though). There's enough scope of the complexity in the modelling of disc mass and the structure of the curves and so on that the findings could be challenged. Their most deviant galaxy is interesting in its own right : visually it looks disturbed, but also lonely. MOND or not, something interesting is definitely going on here.

Galaxies with Declining Rotation Curves

A sample of 22 spiral galaxies compiled from published data is studied. The galaxy rotation curves pass through a maximum distance of more than $\sim 1$ kpc from the center with a subsequent decrease in the rotation velocity.

Don't let your space Nazis die of thirst

It's not all technical critiques of MOND over here. Just occasionally, it's important to step back and think of the bigger picture, like how much water you'd need to supply for an interstellar mission populated by Nazi space milfs.

Haven't got a sodding clue what I'm on about ? Then you must be unaware of my small involvement in a project to simulate the voyage of an interstellar, multi-generational spaceship. This began as an estimate of how many people you'd need to avoid extinction due to inbreeding and whether any breeding controls would be necessary (hence the Nazis), and expanded to consider how much food the population would need. But you can read all about that here.

Having established how many crew you need (about 100 to start, sustainable for millenia at a few times this) and how much food they require, this latest paper looks at air and water. Air consumption depends mainly on mass, whereas water is also strongly temperature dependent, so the code now allows you to set the spaceship temperature.

For this paper, the authors decided to move away from the minimum possible number and use a 1,000-strong crew, about the same as the starship Enterprise (D).  Why this number, I'm not sure, but we're now well into the range of guaranteed survivability. The crew begins as a gender-balanced bunch of thirty-somethings (with a few older and younger) and sets them to have different activity levels (which affect food and water consumption) for different age groups. Temperature varies randomly between 18 and 21 C.

The result of this is that the crew need about 300 tonnes of oxygen and 1,000 tonnes of water per year - just for the humans, never mind the plants and animals. Add in nitrogen to the air and the total air mass is not that far off the water.

It's at this point I have to say I respectfully disagree with the authors on the rest of their approach. Once you establish a minimal level needed per some time period, the next step should be to estimate how this is affected by your recycling capabilities. It's already pretty obvious that our spaceship must be in the range of many millions of tonnes to sustain the crew, but obviously, you want to minimise the mass of water and oxygen as much as possible (especially since we may expect the mass to be totally dominated by the requirements of agriculture). So you could then consider the different cycles in play : how much is lost over different timeframes, e.g. some oxygen is combined with carbon in each breath, plants absorb nitrogen, water is lost rapidly through sweating and breathing but much less frequently through urination. Different recycling procedures will be necessary in all of these, so there will be different recovery timescales and efficiencies at work. Accounting for these would get you a handle on the important number : how much of each you need aboard the ship at any given moment.

Somehow I just wasn't able to persuade the authors of this. So instead they look at other techniques of water and oxygen production via chemical reactions. But these are, I have to say, both unnecessary and counterproductive. Water and oxygen can be stored indefinitely with literally zero risk of contamination, because you're in deep space... and, as we all know :

It is very cold in space.

Star Trek II: The Wrath of Khan (1982) - Yarn is the best way to find video clips by quote. Find the exact moment in a TV show, movie, or music video you want to share. Easily move forward or backward to get to the perfect spot.

So the only effect of producing water and oxygen chemically is to bring along a significant amount of extra mass. You're better off by far simply storing the entire supply needed for the journey and assuming no recycling at all, which, we've established, is already a bad idea. Now I do like the approach suggested of integrating the different processes, which sometimes share different resources and produce outputs the other requires, but this is not much developed, and again, it would be better by far to just recycle. Nor do I understand why they object to bringing in water and other supplies from Solar System bodies prior to the mission - the need to do this is absolutely unavoidable, and the mass of bringing in anything other than pure water must be larger than the optimum case of bringing in pure H2O. It doesn't make any sense.

So how much water will the Nazis need ? Dunno. I can tell you they'll need to use a thousand tonnes per year, but how much they'll actually have to bring along could be completely different. No idea at all. Perhaps future papers will look at that.

Water and air consumption aboard interstellar arks

The architecture of a large interstellar spaceship, which is capable of serving as a living environment for a population over many generations, is mainly dictated by the needs of said population in terms of food, water and breathable gases.

A field guide to mapping the Milky Way

How do you go about mapping the galaxy you happen to live inside of ? There's a hell of a lot of information on GalaxyMap.org, but it's not quite what I'm after. So in this post I'll do a step-by-step guide as to how to construct a map using 21 cm neutral atomic hydrogen data. If you actually want to try this for yourself, I'm going to be assuming some familiarity with Python (especially numpy and astropy/pyfits). Otherwise this post should describe the theoretical aspect well enough to be of interest by itself.


Working in Galactic coordinates

HI data has two main advantages : first, it's not at all subject to extinction by intervening stars and dust, and second, it gives us an easy way to measure velocities. Using trigonometry and a few reasonable assumptions, we can convert velocity into distance, with some limitations.

But before that we need to define a coordinate system. The convention for all-sky HI data is to use Galactic coordinates. In this system, the centre of the Galaxy is defined to be at longitude and latitude of 0. Galactic latitude l and longitude b are defined as follows :

Optical image of the Milky Way overlaid with all-sky data from LAB, with the Magellanic Stream highlighted in orange.

There are two main all-sky Galactic HI surveys : the Leiden-Argentine-Bonn survey and the HI4PI survey*. Both have been gridded in a nice friendly way, such that the pixel size is fixed in latitude, longitude, and velocity. Thus once you know the pixel size in each dimension and the world coordinates of any given pixel, you can very easily calculate the exact coordinates of every other pixel.

* It's 4π steradians, but I always read it to mean "HI For Principal Investigator".

For reference, both surveys have the origin at the bottom left  (l = +180, b = -90). For the LAB survey, the maximum x-pixel (l) range is 720 and the y-pixel (b) range is 360. The pixel size is 0.5 degrees for both axes. For HI4PI, the x and y ranges are 4320 and 2160 respectively, while the pixel size is 5 arcminutes.

And velocity ? For LAB data this spans the velocity range -458.6 km/s (z = 0) to +458.6 km/s (z = 890), with a channel size of 1.03 km/s. For HI4PI the velocity range is -600.0 km/s  (z = 0) to +600 km/s  (z = 945*), with a channel size of 1.288 km/s.

* This value may be slightly off. At the time of writing, I can't access the files I need to check.

Note that these values refer explicitly to the gridded pixel values. These are slightly different from the true resolution values, which are more often quoted in the papers. For the researcher, the real resolution is what matters, but for the data visualiser, it's all about the pixels.


Converting to distance

Once we've found the world coordinates of a pixel, and its flux value, we can then convert this to true 3D position, as follows. First we'll need some assumptions. The Sun is reckoned to be rotating around the centre of the Galaxy with speed V_⊙= 220.0 km/s, at a distance R_⊙= 8.5 km/s. The rotation curve of the Milky Way we can approximate to be totally flat, so that the velocity at any point V_pnt is also always 220.0 km/s. Given the velocity (vel) of any point, we can then calculate is distance R from the Galactic centre :

Note that this further assumes that this is independent of galactic latitude.This is reasonable because the disc is quite thin, but causes problems for structures which are outside the disc completely.

Next we can calculate the distance of the point from the observer :

Where d_1,2 refers to the fact that the equation has two solutions. However, it turns out that this is only really true within side the solar circle, so we don't need to do the calculations twice. Rather we should accept that this region of distance ambiguity is inaccessible to us, so we should only do this calculation if R > R_⊙ (we'll see what happens if we disregard this sage advice later on).

If that's so, we can proceed to calculate Cartesian coordinates of our pixel in PPP (position-position-position) space :

These will be in kpc since those are the units we've been working with. We can now iterate over every pixel in our data set and create a full PPP map from our original PPV (position-position-velocity) cube. This is relatively easy to do, and you can find the Python code to do so for LAB data here (note that some simple extra transform is applied to these final equations, just to ensure the data appears in a sensible position in our PPP cube). We just have to specify the size of the cube we want to make and hardly have to worry about anything else at all. Sounds great ! We'll be using the full information from the original data, so we should get a nice, clean, super detailed map at the end, without even having to specify the pixel resolution or anything even slightly complicated, right ?

Wrong. The problem is that PPV maps to PPP in a very strange way, which is not at all intuitive from the equations (unless you're some sort of trigonometric super freak, I guess). There's no guarantee that every pixel in our PPP cube even corresponds to one in our PPV cube. And not all our PPV pixels will contribute anything, since many of them will lie well outside the Galactic disc where our equations are invalid.

Here's what we get from the LAB data if we do this :

Slice through a PPP cube created from LAB data.

In some regions things are relatively good and we can see some nice astrophysical structures, but other parts are hugely undersampled while others have downright weird artifacts. We can do quite a bit better with HI4PI, which has higher resolution and so more fully samples PPP space, but it's still far from perfect.


Why mapping from PPV to PPP is a bad idea

Let's start with the artifacts. Our observations give us velocity along our line of sight, that is, how fast the gas is moving towards or away from us. In reality, the gas is also moving across the sky, but we can't measure that. We can get these "proper motions" for stars with considerable effort, but we just can't get it for gas.

The first problem is not so much that we'd like to know the proper motion (although that'd be nice), but that the equations assume our line of sight velocity measurements are accurate. But because we're inside the disc of the Galaxy, this is not always true. When we look towards the Galactic centre, or in the opposite direction, the only motion of the gas is across the sky - except for a little bit of random motion (~10 km/s). This means that in those regions of low measured velocity, our data is just too inaccurate for our equations to properly convert line of sight to true velocity. Better instrumentation won't help, it's a fundamental limitation of the structure of the Galaxy and our method.

Velocity vectors relative to the centre in green. Blue and red show the components
towards and away from us, respectively.

The second problem is that the equations have that annoying distance ambiguity within the solar radius, where the equation gives two solutions. Although we might be able to break this degeneracy using other data (e.g. by associating the gas with stars of known distances), by itself there's nothing much we can do to save the HI data. So this region, like the low velocity regions, has to be thrown away.

It might help to visualise how velocity maps to distance. One way of doing this is to plot isovelocity lines : lines of constant velocity.

Being inside the disc has weird consequences for what we can detect and where. Since everything's so darn close, sensitivity is extraordinarily high : we can detect essentially all Galactic gas. Looking through our original data cube, we see tonnes of stuff at very low velocities across the entire sky, because gas at high latitudes is only found when it's close to us and, therefore, moving slowly relative to us, due to the thin nature of the disc. But at the same velocities we can also be detecting material on the far side of the Galaxy !

The bottom line for visualisation is that if we start with the PPV map, we don't necessarily fully sample the PPP cube. This explains the other even more serious problem of the image - all those ugly black lines. How can we fix this ? One answer would be to interpolate extra velocity channels and/or spatial pixels in the PPV cube, so that we'd have more points that map to the PPP data. This does help, but it's inefficient and far from perfect. Even using the enormous HI4PI data set, which has vastly better spatial resolution (though similar velocity resolution) gives only a modest improvement in the sampling.


Alternatively, go directly from PPP to PPV

A much better approach is to work backwards. Beginning with a blank PPP cube, we can calculate the corresponding pixel in the PPV cube and use that to fill in the flux values. This essentially knocks all the problems on the head. By iterating every pixel in the PPP cube, we guarantee that we'll sample the whole thing. Although our calculated pixel positions in the PPV cube won't be integer values, all we have to do is simple rounding and we effectively interpolate the missing data (there are more sophisticated ways to do this, but they can wait for another time).

How exactly do we go about this ? We define the coordinate system of the PPP cube arbitrarily. Then, knowing our Galactic coordinate system, we can use some basic trig to calculate the longitude and latitude of any given pixel. We need to get R first, but this is easy because we know the position relative to the galactic centre gc :

I work in degrees, hence the +90 for convention. The pixel positions xyz must be in physical units (kpc). The tan2 function is a wonderful programmatic convention that simplifies things enormously. Using the usual arctan returns values ±90 degrees, since there's a degeneracy in the tan function. Atan2 gets around this by providing two values, returning values ±180 degrees, which is exactly in accordance with Galactic data gridding conventions (we could convery this easily enough to the range 0-360 if we wanted to, but there's absolutely no need).

All we need now is the line of sight velocity. We can get that by rearranging the earlier equation to calculate R :

Since the original PPV data is gridded in a nice friendly way, the hard part's over. Now that we know the longitude, latitude, and velocity of a pixel in the PPP cube, it's trivial to convert this to the pixel in the PPV cube - remember, the original data has pixel size of constant latitude/longitude/velocity.

Voila. We can now extract the corresponding flux value and create a fully sampled PPP cube.


What to do if your data set is feckin' enormous

But wait ! There's one extra complication. If we want to use the LAB data, we can go right ahead and use the final code. Of course, it's always going to be better to use the HI4PI data, but this is difficult to work with because of its gargantuan 35 GB size. The astropy "pyfits" module is not at all good at dealing with large data sets, so we'll need to convert it into a format it can handle. We can do this using the much older miriad software, which was written from an era when a 100 MB file was considered hefty. Consequently it's massively superior in terms of memory management and can process 35 GB files without breaking a sweat, even on a system with 16 GB RAM.

This bit is trivial. First, we convert the FITS file to miriad's own format using the FITS task. Next, we use the same task to extract individual channels, by setting the region parameter to give single-channel slices (unfortunately, this only works when converting from miriad->FITS, not the other way around, which is why we had to convert the file). Annoyingly, miriad insists on producing FITS files of not one but two channels. We can either accept this and have the script only work with the first velocity channel of each cube (it's super simple to slice the data for this), or first process the files and save ourselves from an extra 35 GB of data we don't actually need. Both steps are easy anyway.

Right. We're pretty much there. We've converted our 35 GB single data file into 944 smaller, more manageable files. All we need to do now is modify our PPP code to work with multiple files - and here it is. As a result of all this, we get the following :

Ta-da ! Lovely. Doesn't quite eliminate all of the artifacts, but we can get rid of those in the visualisation stage.

(For the enthusiast : we could in principle go through every pixel in the PPP cube and open the necessary FITS file every time, but this is hugely inefficient - it means opening the files hundreds of millions of times. I estimated it would take the code 4 months to complete, which made me sad. So instead the code precalculates which pixels it needs to extract for each file. It then orders the list, opens each file, extracts all the pixels it needs from that file, and moves on to the next one. This means it only needs a maximum number of 945 file-opening operations and runs in an hour or so. An extra complication is that a list of hundreds of millions of entries starts to cause memory issues, so the code can work in chunks. The user then has to specify the pixel range of the PPP cube they want to search.)


Making things look pretty

For visuals, we have two options. We can either make a volume render using FRELLED, or we can try and pick out the major features using isosurfaces. Volume renders look prettier and use all of the data, but isosurfaces can make it easier to reveal the important structures and are small enough to display as interactive on-line sections of a web page.

Volume rendering via FRELLED need not be explained in any detail here. Let's skip straight to the render :

Sun position in yellow and the galactic centre in green.

What about isosurfaces ? These are something I've struggled with for a while. Eventually I stumbled on a couple of different Python modules that specialise in this. The one I'm using is scikit-image, which can produce meshes in a format that Blender can recognise. It's also fast and deals with large, complicated meshes pretty darn well. It's not 100% foolproof -  sometimes meshes have their normal vectors pointing the wrong way, but generally this is easy to fix manually.

(I tried other solutions - extensively - like using metaballs and point cloud skinning scripts, and I'd strongly advise everyone else not to try this. They just don't work very well, at best giving ugly results and at worst being useless and inaccurate. Use a dedicated module and save your sanity !)

The code to generate isosurfaces is a bit of a hack at this stage - incorporating it into the next generation of FRELLED is definitely happening, but that's still a ways off. But for now, it works. You'll need this .blend file (containing an internal script and some pre-set materials) and this external Python script, plus this readme file. Eventually I'll make something less hacky, but not today.

Last but not least - exporting to the web. I wanted to use Sketchfab, which I've been very impressed with, but then I discovered it has a stupid 50 MB file size limit. So I spent a weekend investigating Blend4Web, which is free and totally awesome. It's one of those nice things that just works. So here's an interactive 3D model of the HI content of the Milky Way. It'll work on mobiles (it even has a VR mode option !) but it's better on PC - the labels tend to get cut off on a phone. Click on the buttons in the top left panel to toggle different components, and on the annotations for a bit more information.

It's a 28 MB file, may take a few minutes to load, and Blogger won't let me
embed it correctly, so click here for the interactive version.

It's far from perfect yet  - the bloom effect is a bit strong and the anti-aliasing is lousy. But this is my first attempt, so I'm pretty pleased with it. Expect updates on this and lots more interactive content.

So that's it : you now know absolutely everything about how to map the hydrogen content of the Milky Way disc. In a future post I'll look at how to do something similar for the surrounding clouds, which are a lot more fun in 3D because they're found across the entire sky.

A lack of midgets

Ah, another paper on everyone's favourite problem with local cosmology : the chronic lack of dwarf galaxies. By rights, the Local Group should be more crowded with dwarfs than a Smurf convention or the mines of Moria in their heyday. There ought, say simulations, to be hundreds and hundreds of the little hairy sparkly buggers running around orbiting the Milky Way, but there aren't.

An ever-present question as to the significance of this is whether the Milky Way has been visited by the galactic equivalent of Gargamel or if all galaxies have this same problem. Are they all missing their expected dwarfs, or did something peculiar happen to the Milky Way ? Without knowing what things are like for galaxies in general, any purported explanation is largely speculative.

This paper extends this to the nearby spiral M101. At 6 Mpc, it's well within the Local Volume but well outside the Local Group. So if the dwarf-killer is peculiar to our most local environment, M101 shouldn't be affected. From a previous survey they identified potential candidate satellite galaxies of M101, and here they try and establish the distance to the four faintest objects using Hubble.

All four objects appear to be distant background galaxies. Known satellite galaxies at this distance are resolved into individual stars by Hubble observations, but these ones remain stubbornly diffuse. They even simulate what the colour-magnitude diagram for these objects should look like if they were as close as M101, and it's clearly different.

While they don't rule out that future surveys might change their results, since completeness at these very faint magnitudes is always a problem, for now it looks like M101 has an even worse problem than the Milky Way. If the Milky Way doesn't have enough dwarfs, then M101 is positively racist. They also show a comparison with other nearby galaxies where it's possible to measure dwarf abundances with some accuracy (the Milky Way, Andromeda, M94, M81, and Centaurus A). All paint much the same picture.

Wisely, they don't speculate or comment on the astrophysical significance of this. So I'll do their dirty work and point out that this does not constitute any kind of crisis or catastrophe for the standard model. First, we knew this was a problem for the Milky Way anyway, and second, virtually all so-called "problems" for the dark matter paradigm are nothing of the sort : they're problems for the baryonic physics. If Gargamel can't be everywhere at once, it's perfectly plausible that every galaxy has its own Gargamel - a universal property of galaxy formation that prevents star formation in the smallest dark matter halos. Yes, it could also be that the whole dang model is wrong, but much more likely we just don't understand the complicated lives of those pesky little dwarfs. We'll just have to wait and see.

The Satellite Luminosity Function of M101 into the Ultra-Faint Dwarf Galaxy Regime

We have obtained deep Hubble Space Telescope (HST) imaging of four faint and ultra-faint dwarf galaxy candidates in the vicinity of M101 - Dw21, Dw22, Dw23 and Dw35, originally discovered by Bennet et al. (2017).